It is a well known fact that the more a combustion engine provided with opened valves in the cylinder head is loaded, and the higher will the cylinder pressure be when the outlet valves are to be opened and the more energy will be required for the opening thereof. This fact is regardless of the method used for the opening of the valves. A problem encountered by a pneumatically, electromagnetically or hydraulically activated valve opening, but not encountered by cam shaft activated valve opening, is the opening against high cylinder pressures upon evacuation of combustion gases. The main problem is that the dimensioning of the valve opening power has to be adopted to the highest cylinder pressure instead of the most common cylinder pressure, which is substantially lower than the highest pressure. If the operation of the valve is done by a means such as a pressure fluid or an electromagnet, the power of this means must, with contemporary technology, be dimensioned in order to be able to perform a valve opening against the force generated by the maximum cylinder pressure. It would be desirable to reduce this need of over dimensioning.
Furthermore, a high pressure in the cylinder will, when the exhaust gases are evacuated, result in a waste of energy in the sense that a possible expansion work is not made use of (however if there is a turbo, some of the energy will be made use of).
The motor-braking technique for vehicles can be developed as to capacity and controllability, and for power-saving purposes. In vehicles with combustion engines such as heavy lorries, motor-braking is actively used as a supplement to the ordinary braking system. One such method comprises exhaust gas-braking, by which the evacuation of the exhaust gases out of the cylinder of the engine is inhibited. As an alternative, air is permitted to leak out into the exhaust gas system during the compression strokes. A problem of the methods as of today is the fact that a maximum braking effect and the possibility of controlling the braking effect is unsatisfactory. In connection to this problem, and with relevance for light vehicles, there is a possibility of storing brake energy upon motor-braking. There are known suggestions of how to conduct compressed air to a tank upon motor-braking, and to use the stored energy when there is a subsequent need of acceleration. There is a common feature of both light weight and heavy weight vehicles that a good controllability and high brake efficiency and energy saving by means of compressed air is enabled through the use of freely operable valves in the cylinder head of the engine. In this context, upon use of freely operable valves, it might be a problem that the cylinder pressure will be high during compression strokes, resulting in a need of a large amount of energy for the purpose of opening the valves. A large engine load is another operation mode that presents a relatively high cylinder pressure when the exhaust gases are to be evacuated.